1. Field of the Invention
The present invention relates to a method and an apparatus of welding inspection.
2. Description of Related Art
Welding is in heavy usage for assembly processes of automobiles. A reason for the heavy usage of welding is that structural members made by matching edges of thin steel plates and welding the matched edges are used in quite a few cases in such assembly processes. For example, a frame of a truck is made by bending a thick steel plate into a horseshoe shape. Used for a cabin and a roof in this case are structural members with their cross sections of a square pipe made of a combination of bent thin steel plates. As described above, structural members made of thin steel plates press-bent and welded are used in a great number in manufacturing processes of automobiles.
An inspection method using ultrasonic waves is described below with reference to FIG. 1 through FIG. 9, as a conventional welding inspection method in relation to the present invention. FIGS. 1 and 2 are drawings that explain the conventional welding inspection method, and show a positional relationship between a conventional ultrasonic probe 10 (hereinafter, simply called a “probe”) and a combination of steel plates 13-1 and 13-2 as specimens. FIG. 3 is a drawing for showing a welded part 12 (hereinafter, called a “nugget”). Though spot welding is described in the following explanation, a scope of application of the present invention is not limited to the spot welding.
The probe 10 includes a transducer 11, which is equipped with a sending/receiving element for ultrasonic waves. The transducer 11 is connected to a welding inspection apparatus, which is not shown in the drawing; and the transducer 11 sends and receives an ultrasonic wave as a pulse signal. The welding inspection apparatus receives a reflected ultrasonic wave at each time of sending a pulse signal in order to measure the intensity of the reflected ultrasonic wave.
Furthermore, the probe 10 includes a contacting part 14, which is internally charged with a liquid such as water. Thus, an ultrasonic wave launched from the transducer 11 can efficiently be transmitted to the steel plate 13-1. The contacting part 14 is formed with an elastic material such as rubber so that the condition of contact between the probe 10 and the steel plate 13-1 does not change even when a posture tilt of the probe 10 changes somewhat.
As shown in FIG. 3, the nugget 12 is formed at a boundary plane between the plurality of steel plates 13-1 and 13-2 being stacked. In an example of FIG. 3, the two steel plates 13-1 and 13-2 are stacked. However, the number of stacked steel plates may be more than two under the same structural concept.
As shown in FIGS. 1 and 2, in the conventional welding inspection method using an ultrasonic wave, the probe 10 needs to be placed right above the nugget 12, namely to be placed right on a spot-welded section 15.
A reason for such an arrangement described above is that, for materialization of the inspection, an ultrasonic wave launched from the probe 10 must indispensably pass through the nugget 12, meanwhile the probe 10 must receive a reflected wave of the ultrasonic wave. Therefore, the probe 10 is necessarily placed right on the spot-welded section 15. Furthermore, to efficiently receive the reflected ultrasonic wave, a reflection plane for the ultrasonic wave needs to be almost perpendicular to a traveling direction of the ultrasonic wave, and accordingly the probe 10 requires an adequate accuracy on a tilt adjustment.
FIGS. 4 to 9 are drawings for explaining a concrete example of the conventional welding inspection method using an ultrasonic wave. FIG. 5 shows a graph of an inspection result of the nugget 12 that is made up normally; and in the graph, a horizontal axis and a vertical axis are a time axis and axis of an intensity of the reflected ultrasonic wave, respectively.
The ultrasonic wave launched toward the steel plate 13-1 from the probe 10 travels back and forth several times between a top surface of the steel plate 13-1 and a bottom surface of the steel plate 13-2 in a certain time period, and eventually an energy of the ultrasonic wave is attenuated to disappear. While the ultrasonic wave travels back and forth within the steel plates 13-1 and 13-2, a part of the ultrasonic wave reflected between the top surface of the steel plate 13-1 and the bottom surface of the steel plate 13-2 returns to the probe 10. An example of a waveform image of the ultrasonic wave for enabling an inspector to visually check it by using an image display apparatus (an oscilloscope) is shown in FIG. 5, the ultrasonic wave having returned to the probe 10.
FIG. 5 shows a waveform of an ultrasonic wave in the case where the nugget 12 is made up normally. In this case, several repetitive waveform peaks according to thicknesses of the steel plates 13-1 and 13-2 appear; and the repetitive waveform peaks result in a certain attenuation curve, and such a waveform pattern is output. A quality judgment on the nugget 12 is carried out through evaluation of the number of waveform peaks, a level of each waveform peak, a distance between two neighboring waveform peaks, presence of any waveform peak that should not appear primarily, and others.
A speed of the ultrasonic wave traveling in the nugget 12 is constant. Therefore, a plurality of reflected ultrasonic waves, which are reflected at the bottom surface of the steel plate 13-2, appear at intervals of transmission time of the ultrasonic waves, corresponding to the thicknesses of the steel plates 13-1 and 13-2. Accordingly, the horizontal axis in FIG. 5 is a time axis that represents a distance in a thickness direction of the steel plates 13-1 and 13-2 (for example, refer to Patent Document 1).
On the contrary, FIG. 6 shows another case in which the nugget 12 is not made up normally, having a gap or pealed between the steel plates 13-1 and 13-2. FIG. 7 shows a graph of an inspection result of the nugget 12 that is a defective one having the pealed; and in the graph, a horizontal axis and a vertical axis are a time axis and axis of an intensity of the reflected ultrasonic wave, respectively.
Under a condition where the nugget 12 is not made up normally, having a pealed between the steel plates 13-1 and 13-2, as shown in FIG. 7, there appear repetitive waveforms with a less attenuation due to the plate thickness at a searching surface side. Namely, if there exists a pealed between the steel plates 13-1 and 13-2, an ultrasonic wave launched from the probe 10 travels back and forth only between the top surface and a bottom surface of the steel plate 13-1. Accordingly, in comparison with the example shown in FIG. 5, a traveling distance of the ultrasonic wave is shorter so that peak intervals become shorter than those shown in FIG. 5. Furthermore, the short traveling distance of the ultrasonic wave results in a less attenuation in comparison with the example shown in FIG. 5 so that peaks having the same intensity consecutively appear at short intervals. When such peaks appear, it is determined that there exists a pealed between the steel plates 13-1 and 13-2.
FIG. 8 shows still another case in which the nugget 12 is made to be small. FIG. 9 shows a graph of an inspection result of the nugget 12 that is made to be small; and in the graph, a horizontal axis and a vertical axis are a time axis and axis of an intensity of the reflected ultrasonic wave, respectively.
Under a condition where the nugget 12 is made smaller than in a normal case, there appears a condition equivalent to including two modes in parallel within an inspection area of the probe 10; namely, one mode with the nugget 12 made up normally as shown in FIG. 5 and the other mode having the pealed shown in FIG. 7. Accordingly, in a waveform there appear together the large peaks shown in FIG. 5 as well as the small peaks shown in FIG. 7. The small peaks existing between the large peaks are distinctively called “Napoleon hats”. When such Napoleon hats appear, it can be determined that the nugget 12 is made smaller than in a normal case.
As described above, in the conventional welding inspection method, the probe 10 for executing the inspection is placed right above the nugget 12 to be almost perpendicular to the steel plate 13-1.